Submerged evaporator with integrated heat exchanger

ABSTRACT

The invention provides a submerged evaporator ( 14 ) with integrated plate heat ex-changer ( 4 ) that may operate with markedly increased capacity, where the evaporator ( 14 ) does not require more space than other known types and have a smaller filling volume of refrigerant ( 10 ) than prior art units. The integrated plate heat exchanger ( 4 ) is built up with an outer contour that substantially follow the lower contour of the casing ( 6 ) and the liquid level, where the integrated heat exchanger ( 4 ) has at least one inlet connection ( 24.1 ) and at least one outlet connection ( 24.2 ) for secondary refrigerant ( 26 ), and where the upper volume of the casing acts as a liquid separator. With such design, much less space is occupied than with prior art types of submerged evaporators. The reason is that the internal volume is better utilised. Typically, there is a cylindric casing ( 6 ) with welded or screwed on ends ( 22 ), where internally there is mounted a plate heat exchanger ( 4 ) having a part cylindric shape and an external diameter which is between 5 and 15 mm less than the internal diameter of the casing ( 6 ). Hereby is achieved a submerged evaporator ( 14 ) with reduced filling of refrigerant ( 10 ).

The present invention concerns a submerged evaporator contained in acasing and including at least one submerged plate heat exchanger, wherethe submerged plate heat exchanger has at least one inlet connection andat least one outlet connection for a secondary refrigerant, where theplate heat exchanger is disposed at the bottom of the casing, where aprimary refrigerant may flow around the plate heat exchanger and asecondary refrigerant may flow through the plate heat exchanger, andwhere the uppermost part of the casing is used as a liquid separator.

Using a submerged evaporator is a known method of heat transmissionbetween two separate media. One of the commonly known methods is toincorporate a cylindric plate heat exchanger in a cylindric casing.Above this casing is mounted a liquid separator typically having thesame size as the casing enclosing the plate heat exchanger. Thissolution has, among others, the drawback that relatively much space isoccupied in height simultaneously with, due to the height of the unit,there being a large static pressure suppressing the evaporation,particularly at lower temperatures, thus reducing efficiency.Furthermore, a pressure loss occurs between evaporator and the separateliquid separator, also reducing capacity.

EP 0 758 073 describes a refrigeration device in a closed refrigerantcircuit for cooling a cold transfer medium, in particular a water/brinemixture, in the refrigerant circuit a compressor sucking in gaseousrefrigerant from a vapour drum, compressing the said refrigerant andsupplying it at high pressure to a condenser, from which, after pressureexpansion, the liquid refrigerant is supplied via the liquid space ofthe vapour drum to an evaporator, in which heat is extracted from thecold transfer medium as a result of the evaporation of the refrigerant,and from which the gaseous refrigerant is supplied once again to thevapour space of the vapour drum, the heat exchanger surface of theevaporator being designed as a plate heat exchanger with media conveyedin cross-current and counter-current to one another and being arrangedin the liquid space of the vapour drum, where the heat exchanger surfaceof the plate heat exchanger is submerged into the vapour drum, designedas a pressure-resistant housing, in such a way that the supplyconnection piece and the discharge connection piece are arranged on oneside and the deflection chamber for the cold transfer medium flowinghorizontally through the plate heat exchanger is arranged on the otherside, outside the housing of the vapour drum, and defining fall ductsfor the refrigerant circulated by natural circulation as a result ofgravity are formed between the two side walls of the plate heatexchanger and the housing walls of the vapour drum which are parallelthereto.

In this solution part of the heat exchanger is placed outside the vapourdrum. Different parts of the heat exchanger are subjected to differentpressures; the part outside the drum is subjected to atmosphericpressure, where the part inside the drum is subjected to the evaporationpressure inside the drum. Depending on the cooling media used, thepressure difference can be very high. The heat exchanger is box-shaped,and that form leaves a lot of unused space around the box especiallyunder the box and along the two sides. This space takes up a largevolume of unused cooling media. The strength of the box-shaped heatexchanger is not sufficient if a high pressure difference occurs. In oneembodiment, the passive volume is reduced by out filler volumes placednear the bottom of the drum. The static pressure around the heatexchanger is relatively high because of the upright drum, and the staticpressure reduces evaporation because steam bubbles formed by evaporationhave a reduced sizes.

U.S. Pat. No. 4,437,322 describes a heat exchanger assembly for arefrigeration system. The assembly is a single vessel constructionhaving an evaporator, condenser and flash subcooler. A plate inside theshell separates the evaporator from the condenser and the flashsubcooler, and a partition inside the vessel separates the condenserfrom the flash subcooler. The heat exchanger assembly includes acylindrical shell having a plurality of tubes disposed in parallel tothe longitudinal axis of the cylindrical shell.

By placing the tubes inside the shell, there is no pressure differentialover the heat exchanger, but the heat exchanger has a reduced surface asformed by longitudinal tubes. Over the heat exchanger there is only alimited space, and a small amount of liquid refrigerant might be suckedout of the vessel.

A heat exchanger assembly is also disclosed in U.S. Pat. No. 4,073,340.A heat exchanger of the shaped plate type with a stack of relativelythin interspaced heat transfer plates. The plates of the heat exchangerare arranged to define sets of multiple counterflow fluid passages fortwo separate fluid media alternating with each other. Passages of oneset communicate with opposed manifold ports on opposite sides of thecore matrix. Passages of the other set pass through the stack past themanifolds in counterflow arrangement and connect with inlet and outletportions of an enclosing housing. An assembly of two plates oppositelydisposed establishes integral manifolds for one of the fluid mediathrough the ports and the fluid passage defined between the plates. Athird plate joined thereto further defines a passage for the secondfluid media to flow between the inlet and outlet portions of thehousing. The various fluid passages may be provided with flow resistanceelements, such as baffle plates, to improve the efficiency of heattransfer between adjacent counterflow fluids. In each set of alignedports, collars, alternately large and small, are formed in nestedarrangement so that the ports formed by adjacent plates bridge the innerspaces between the plates. Such construction permits communication withthe aligned ports of alternate fluid channels which are closed to theoutside between the heat exchanger plates. In manufacturing a corematrix, the parts are formed and cleaned and the brazing alloy isdeposited thereon along the surfaces to be joined. The parts are thenstacked in the natural nesting configuration followed by brazing in acontrolled-atmosphere furnace. The brazing is readily carried out due tothe sealing construction of the described nesting arrangement.

This heat exchanger is designed for air to gas heat exchange. If theplates are used inside an evaporator, the shape of the plates leads to acasing containing a large volume of unused refrigerant.

The invention described in WO 97/45689 concerns a heat exchanger whichhas a plate stack and comprises first and second plates which arearranged alternately in rows and between which first and second channelsare formed, these channels being connected via first and secondconnection regions to first and second connection openings. The firstconnection openings, first connection regions and first channels arecompletely separate from the second. The first and second plates eachhave on both sides a plurality of substantially straight main channelswhich are aligned in parallel in each plate. The first channels andsecond channels consist of first and second main channels and third andfourth main channels which mutually form a first angle and are formed onboth sides of a first connection plane and a second connection plane inthe form of half channels which are open towards the connection plane.The fourth main channels and second main channels are formed on one sideof a first plate and second plate, and the first main channels and thirdmain channels are formed on the other. The plates are metal sheets whosemain channels on both sides take the form of beads which appear on oneside of the metal sheet as depressions and on the other as burr-likeprojections. On one side of the metal sheet, a contact surface isprovided along the periphery, and, on the other, two contact regions,each enclosing a passage opening, are provided, so that, by joiningtogether the metal sheets with the same sides or planes in each case,contact surfaces and contact regions always alternately abut one anotherand are tightly interconnected, in particular welded or solderedtogether, in order to separate the first and second channels in a leaktight manner.

These problems have been attempted solved in another known type where inone and the same casing a plate heat exchanger and a liquid separatorare incorporated. This is e.g. disclosed in U.S. Pat. No. 6,158,238.Here is described a heat exchanger which is built up with a cylindriccasing having a diameter, which is markedly greater than the diameter ofthe built-in cylindrical plate heat exchanger, whereby the plate heatexchanger disposed at the bottom of the casing may be submerged byprimary refrigerant while there is still space for a liquid separatorfunction. This solution provides a relatively low static pressure, andno pressure drops problems between evaporator and liquid separator arepresent either as they are built together. This kind of submerged plateand casing heat exchanger, however, has the great disadvantage that avery large and in many cases unacceptable filling of the primaryrefrigerant is required, where a large part of the filling is actuallyjust passive and uselessly provided between casing and plate heatexchanger. The efficiency of the system compared with space requirementsis also not optimal since by this design there is needed a casing with adiameter which is often in the range 1.5-2 times the diameter of thebuilt-in plate heat exchanger.

Another and very significant disadvantage of the above systems is thatmixing occurs in the primary refrigerant between the upwards directedflow coming from evaporation of the primary refrigerant and therefrigerant in liquid state which is on its way back to the bottom ofthe casing. At the bottom of the casing may hereby occur a lack ofrefrigerant whereby the efficiency is considerably reduced.

It is the purpose of the invention to indicate a submerged evaporatorwith integrated plate heat exchanger that can operate with a markedlyincreased capacity compared with prior art heat exchangers, where theheat exchanger does not require more space than prior art evaporators,and furthermore where there is need for a considerably less fillingvolume of the primary refrigerant than in prior art units.

This may be achieved with a submerged evaporator with integrated plateheat exchanger as described in the introduction, where the plate heatexchanger is integrated in the liquid separator, and where theintegrated plate heat exchanger is made with an outer contour thatsubstantially follows the lower contour of the casing and the surface ofthe liquid level of the primary refrigerant.

With such a design of the plate heat exchanger, the size of the entireevaporator may be optimised so that substantially less space is occupiedthan by prior art types of submerged evaporator with the same capacity.The primary reason for this is that the internal volume is utilisedbetter. A submerged evaporator of this type furthermore has a minimalstatic pressure and a minimal pressure loss between evaporator andliquid separator and of course a substantially less filling than atraditional evaporator with the same capacity. The integrated plate heatexchanger is made with a shape following the internal contour of thecasing. Typically, we are speaking of a traditionally shaped cylindriccasing with welded or screwed ends where internally there is fitted aplate heat exchanger having a partly cylindric shape, e.g. asemi-cylindrical shape, and an outer diameter which is 5-15 mm less thanthe inner diameter of the casing. With this design, there is achieved asubmerged evaporator with a markedly reduced filling of primaryrefrigerant. In order to attain maximum effect of the submergedevaporator, it is, as indicated, to be submerged, and with a submergedevaporator according to the invention, only a limited volume is requiredas only a minimal waste volume is present, i.e. no large passive areasbetween the sides of the heat exchanger and the casing are to be filledby the primary refrigerant.

In an embodiment of the invention, a submerged evaporator withintegrated plate heat exchanger is designed so that the longitudinalsides of the plate heat exchanger are closed for inflow or outflow ofthe primary refrigerant between the plates of the plate heat exchanger,and that in the bottom of the plate heat exchanger there is provided atleast one opening through which the primary refrigerant flows in betweenthe plates of the plate heat exchanger. With these closed sides isachieved the advantage that liquid carried with the evaporatedrefrigerant can be conveyed back to the bottom of the plate heatexchanger without mixing evaporating refrigerant and unevaporatedrefrigerant liquid on its way back to the bottom of the evaporator againis occurring.

In a preferred variant of the invention, longitudinal guide platesextending from an area in the vicinity of the top side of the plate heatexchanger and downwards against the bottom of the casing are provided inlongitudinal gaps appearing between plate heat exchanger and casing,where the downwardly extension of the guide plates has a magnitude sothat a longitudinal area at the bottom of the plate heat exchanger isheld free from guide plates, where the primary refrigerant may flow inbetween the plates of the plate heat exchanger. By this design is alsoachieved that the downwardly flowing liquid is not admixed with upwardlyflowing liquid, whereby the efficiency of the submerged evaporator withintegrated heat exchanger is increased significantly.

In a further embodiment of the invention, a submerged evaporator has aplate heat exchanger built up of plates that are embossed with a patternof guide grooves pointing towards the inner periphery of the casing atthe upper edge of the plates with an angle between 0° and 90° inrelation to level, and preferably with an angle between 20° and 80°.With these guide grooves is achieved a more rapid and more optimalleading back of unevaporated refrigerant as the refrigerant is conductedtowards the inner periphery of the casing and then flows down along thesides of the casing and back to the bottom of the plate heat exchanger.In this way, the liquid separating action is enhanced since hereby isensured that possible liquid carried with remains in the liquidseparator/casing.

A submerged evaporator with integrated heat exchanger may furthermoreinclude a condenser designed as a plate heat exchanger, which is mountedin the “dry” part of the casing, and which is separated from theevaporator section by a plate. Hereby is achieved possibility ofperforming condensing of the evaporated refrigerant or a part thereof.

Furthermore, a submerged evaporator with integrated plate heat exchangermay include a demister (drip-catcher), where the demister is mounted inthe casing in immediate vicinity of the outlet connection for evaporatedrefrigerant. By such a demister it is possible to remove unwanted dropsof unevaporated refrigerant before the vapour leaves the evaporator, andat the same time it is possible to minimise the size of the casing andstill have the same capacity.

A submerged evaporator according to the invention may be adapted so thatsecondary refrigerant may flow to and from the plate heat exchanger viaone inlet connection and one outlet connection, respectively, at theupper edge of the plates. Alternatively, the secondary refrigerant mayflow to and from the plate heat exchanger via one connection at thebottom of the plates and one connection at the upper edge of the plates,respectively. A further alternative is that secondary refrigerant mayflow to and from the plate heat exchanger via one connection at thebottom of the plates and two connections at the upper edge of theplates, respectively. With these connection possibilities, such asubmerged evaporator may be adapted to many different operatingconditions, where the different connecting arrangements may beassociated with advantages for different reasons. Direction of flow maybe chosen freely, depending on the actual operating conditions.

Finally, a submerged evaporator according to the invention may include asuction manifold disposed in the “dry” part of the casing and extendingin longitudinal direction of the evaporator with a length substantiallycorresponding to the length of the plate heat exchanger. This manifoldhas the effect that, due to even suction of the gases, the liquidseparation action is improved, and the size of the casing may be kept ata minimum level and possibly be reduced.

In the following, the invention is described with reference to thedrawing, which, without being limiting, shows a preferred embodiment ofa submerged evaporator according to the invention, where:

FIG. 1 shows the prior art type of submerged evaporator with submergedplate heat exchanger,

FIG. 2 show a cross-section of a submerged evaporator with integratedplate heat exchanger according to the invention as seen from the end,

FIG. 3 shows a submerged evaporator seen from the side,

FIG. 4 shows position of guide plates,

FIG. 5 shows possible design of guide grooves in the plates of the heatexchanger,

FIG. 6 shows a submerged evaporator with integrated condenser anddemister,

FIG. 7 shows different connecting possibilities for the secondaryrefrigerant, and

FIG. 8 shows a section through a part of the heat exchanger.

On FIG. 1 is seen a prior art submerged evaporator 2 with submergedplate heat exchanger 4. The casing 6 has a diameter which is typically1.5 to 2 times larger than the diameter of the cylindric plate heatexchanger 4, which is necessary since the cylindric plate heat exchanger4 is to be covered with the primary refrigerant liquid 10 while at thesame time sufficient space is to remain for the liquid separatorfunction. As a natural consequence of the diameter difference betweenthe plate heat exchanger 4 and the surrounding casing 6, a relativelylarge volume is provided at the sides 8 of the heat exchanger, filledwith primary refrigerant 10. This large volume is, however, alsonecessary in order to ensure that not too much mixing occurs between therefrigerant 10, which is on its way down to the evaporator bottom 12,and the refrigerant 10, which is brought to evaporate between the platesof the plate heat exchanger.

FIG. 2 shows a submerged evaporator 14 with integrated plate heatexchanger 4 according to the invention, where it is clearly seen thatthe heat exchanger 4 almost entirely fills the submerged part of thecasing 6, and thus does not require so large filling with primaryrefrigerant 10 as with the prior art. The cross-section shown hereillustrates that the heat exchanger 4 has a semi-cylindricalcross-section, but may of course be made with any conceivable kind ofpart cylindric cross-section or with another shape utilising the actualshape of the casing 6 optimally. Typically, the plate heat exchanger 4may be provided with a cut-off or flat bottom 16 as depicted on FIG. 4.

On FIG. 3 is seen the same unit as on FIG. 2, but here in a longitudinalsection of the unit 14, i.e. in a side view. On this Figure is seen asuction manifold 18 disposed inside the casing 6 in the dry part 20constituted by the liquid separator. This manifold 18 provides anoptimised utilisation of the evaporated refrigerant 10 and thereby anincreased efficiency. At the end of the casing 6 is seen the lead-in ofthe connecting connections 24 where the secondary refrigerant 26 isconducted into and out of, respectively, the integrated plate heatexchanger 4. The direction of flow may be chosen freely depending ondiverse conditions.

The integrated plate heat exchanger 4 may, as mentioned previously, beequipped with guide plates 28 between the sides of the heat exchanger 4and of the casing 6. An example of placing guide plates 28 appears onFIG. 4. Moreover is seen that the casing 6 may be reinforced with one ormore horizontal braces 30 fastened between the end plates 22. Analternative solution for ensuring that refrigerant 10, which is on itsway back to the bottom 12 of the casing 6, is not mixed with and carriedon by evaporated refrigerant 10, is welding of individual plates 34along the sides 8 of the plate heat exchanger; alternatively, theindividual plates may be designed so that they, in mounted condition,are lying closely together, whereby the same effect is attained. Withthis solution is ensured a passage 32 between heat exchanger 4 andcasing 6, where refrigerant 10 may flow freely towards the bottom 12 ofthe casing 6. At the bottom 12 of the plate heat exchanger there is, ofcourse, free access between the plates 34 so the primary refrigerant 10may flow in between the plates 34 and be brought to evaporate.

The individual plates 34, which the plate heat exchanger 4 is made upof, are normally embossed with a pattern called guide grooves 36, seeFIG. 5, and having the purpose of ensuring a more optimal heat transferas well as contributing to respective refrigerants 10 being conductedoptimally through the heat exchanger 4. At the upper edge 44 of the heatexchanger plates 34, these grooves 36 typically are directed against thecasing 6 with an angle between 0° and 90°, and on FIG. 5 the angle isabout 60° in relation to level. It is apparent that this angle may vary,depending on the design of the rest of the system. Also, it is clearthat the direction of the mouth of these guide grooves 36 does notnecessarily have any connection to the way in which the grooves 36 aredesigned in the remaining area of the plates 34. As previouslymentioned, this design is determined from heat transmission aspects.

On FIG. 6 is seen a variant of a submerged evaporator 14 with integratedplate heat exchanger 4. In this variant, there is furthermore mounted acondenser 38 which in principle is designed as a plate heat exchanger 4submerged at the bottom 12 of the casing 6, but mounted in the “dry”part 20 of the casing 6, and separated from the evaporator section by aplate. This plate may alternatively be constituted by welded platecassettes in the condenser. The evaporator 14 shown on FIG. 6 isfurthermore equipped with a demister 40 mounted in the casing 6 underthe outlet 42 for evaporated refrigerant 10.

On FIG. 7 are seen three different possibilities for connecting 24piping for the secondary refrigerant 26. FIG. 7.1 shows inlet 24.1 atthe right side and outlet 24.2 at the left side of the plate heatexchanger 4, and FIG. 7.2 shows inlet 24.1 at the bottom 12 of the plateheat exchanger 4 and outlet 24.2 in the top 44 at the middle. Finally,FIG. 7.3 shows inlet 24.1 at the bottom 12 as shown on FIG. 7.2, buthere there are two outlet connections 24.2 at the upper edge 44 cornersof the heat exchanger 4. The shown connection possibilities are justexamples and are not in any way to be viewed as limiting for the choiceof connection arrangement. The secondary refrigerant may be single phasebut may e.g. also be a condensing gas.

On FIG. 8 is shown a section through a part of a submerged evaporatorsurrounded by a casing 6. Inside the evaporator are shown heat exchangerplates 34 between which there is shown volumes containing the primaryrefrigerant 10 and volumes containing the secondary refrigerant 26.Between the casing and the heat exchanger plates 34 there are formedducts 32 in which primary refrigerant is flowing.

Heat transmission occurs from the secondary refrigerant 26 to theprimary refrigerant 10, whereby the primary refrigerant 10 is heated toa temperature above the boiling point of the medium. Therefore, boilingwith development of steam bubbles in the primary refrigerant 10 occurs.These steam bubbles seek upwards in the ducts formed between the plates34 of the heat exchanger. Simultaneously, the rising bubbles result inan upward liquid flow, increasing the efficiency of the evaporator. Atthe same time, the upward flow results in a downward flow in the ducts32, where the primary refrigerant 10 flows downwards, primarily onliquid form. Thereby is ensured an efficient flow around and through theducts of the evaporator.

1. A submerged evaporator contained in a casing and including at leastone integrated plate heat exchanger, where the integrated plate heatexchanger has at least one inlet connection and at least one outletconnection for a secondary refrigerant, where the plate heat exchangeris disposed at the bottom of the casing, where a primary refrigerant mayflow around the plate heat exchanger and a secondary refrigerant mayflow through the plate heat exchanger, and where the uppermost part ofthe casing is used as a liquid separator, wherein the integrated plateheat exchanger is integrated with the evaporator and made with an outercontour that substantially follows the lower contour of the casing andthe liquid level of the primary refrigerant.
 2. A submerged evaporatoraccording to claim 1, wherein the longitudinal sides of the plate heatexchanger are closed for inflow or outflow of the primary refrigerantbetween the plates of the plate heat exchanger, and wherein the bottomof the plate heat exchanger there is provided at least one openingthrough which the primary refrigerant flows in between the plates of theplate heat exchanger.
 3. A submerged evaporator according to claim 1,wherein longitudinal guide plates extending from an area in the vicinityof the top side of the plate heat exchanger and downwards against thebottom of the casing are provided in longitudinal gaps appearing betweenplate heat exchanger and casing, where the downwardly extension of theguide plates has a magnitude so that a longitudinal area at the bottomof the plate heat exchanger is held free from guide plates, where theprimary refrigerant may flow in between the plates of the plate heatexchanger.
 4. A submerged evaporator according to claim 1, wherein theplates of the plate heat exchanger are embossed with a pattern of guidegrooves pointing towards the inner periphery of the casing at the upperedge of the plates with an angle between 0° and 90° in relation tolevel, and preferably with an angle between 20° and 80°.
 5. A submergedevaporator according to claim 1, wherein, including a condenser shapedas a second plate heat exchanger, which is mounted in the “dry” part ofthe casing, and which is separated from the evaporator section by aplate.
 6. A submerged evaporator according to claim 1, wherein includinga demister which is mounted in the casing in immediate vicinity of theoutlet connection for evaporated refrigerant.
 7. A submerged evaporatoraccording to claim 1, wherein being adapted in order that secondaryrefrigerant may flow to and from the plate heat exchanger via one inletconnection and one outlet connection, respectively, at the upper edge ofthe plates.
 8. A submerged evaporator according to claim 1, whereinbeing adapted in order that secondary refrigerant may flow to and fromthe plate heat exchanger via one connection at the bottom of the platesand one connection at the upper edge of the plates, respectively.
 9. Asubmerged evaporator according to claim 1, wherein being adapted inorder that secondary refrigerant may flow to and from the plate heatexchanger via one connection at the bottom of the plates and twoconnections at the upper edge of the plates, respectively.
 10. Asubmerged evaporator according to claim 1, wherein the casing contains asuction manifold disposed in the “dry” part of the casing and extendingin longitudinal direction of the evaporator with a length substantiallycorresponding to the length of the plate heat exchanger.